The long term objective of this project is to understand the encoding of complex stimuli in the auditory nervous system. Three questions related to previous studies of encoding in the auditory nerve will be addressed by the research proposed: 1) How is the auditory-nerve representation of a complex stimulus transformed by the cochlear nuclei? 2) Are the encoding schemes which have been worked out in anesthetized preparations valid for the awake behaving animal and how do efferent effects in the cochlea affect the encoding? 3) What is the relationship between the auditory-nerve representation of sounds and the cat's ability to discriminate them? Specifically, the encoding of complex stimuli in populations of cochlear nucleus output cells will be studied, first in the decerebrate cat and then in an awake behaving cat preparation. In the anesthetized cat, dynamic ranges of most auditory-nerve fibers are limited to about 50 dB. This dynamic range limitation imposes constraints on stimulus encoding and it has been suggested that efferent influences in the cochlea and/or cochlear nuclei may extend the dynamic range in a behaving cat. Dynamic range of cochlear nucleus cells will be studied in cats which are engaged in an auditory discrimination task. The mechanism of action of the efferent input to the cochlea will be studied in anesthetized cats by electrically stimulating the efferent system in the brain stem and measuring the effects on auditory-nerve fiber rate functions for one-and two-tone stimuli. Finally, discrimination functions for tones in noise will be measured for cats with permanently implanted round window electrodes. Comparison of evoked potentials from the behaving cats with those from anesthetized cats will indicate efferent effects in the cochlea. These studies have direct relevance to attempts to develop multichannel cochlear implants in that they define the encoding in the normal auditory nerve and begin to look at how that encoding is transformed in the central nervous system.